omaha: remove XMLName from nested structures
omaha: remove XMLName from nested structures

The special XMLName field is only useful in the top level structs which
need something to attach the lower case tag to. On the rest the default
behavior without XMLName works just fine so it is clutter.

The datastore tags have been dropped too since they are not needed.

quantum/coreos-mantle,marineam/mantle,dm0-/mantle,glevand/coreos--mantle,mischief/mantle,mischief/mantle,mjg59/mantle,quantum/coreos-mantle,mjg59/mantle,dm0-/mantle,glevand/coreos--mantle,marineam/mantle